Laser scanning devices, such as laser plotters, are known in the art. As shown in FIG. 1, to which reference is now made, laser scanning devices typically comprise a laser source 10 which produces a laser beam 12 and a spinner 14 which receives the laser beam 12 after it has passed through a pre-spinner optical system 16 operative to shape and/or modify the laser beam 12.
The spinner 14 comprises a motor (not shown) and a polygon typically having a plurality of facets 18, only one of which generally is operative at a given time to reflect the laser beam 12 towards a medium 22 to be scanned. Each facet 18 is operative to scan at least one line of the medium 22 and the angle through which each facet 18 scans is indicated in FIG. 1 by the dotted lines, marked 24.
In some devices, a post-spinner optical system 20 is also included which typically comprises a flat-field lens operative to provide a planar image on the medium 22 (the focal point of the lens) and to direct the laser beam 12 towards the medium 22, typically via a mirror if the medium 22 is not, as shown in FIG. 1, parallel with a rotation axis 26 of the spinner 14 (perpendicular to the page of FIG. 1).
In order for a laser scanning device to operate at high accuracy and resolution it must accurately control the location of the laser beam, along the scan direction and along the direction perpendicular to the scan direction, the direction known as the "cross-scan" direction.
As described in Chapter 6 of the book Optical Scanning by Gerald F. Marshall, Marcel Dekker, Inc. N.Y., 1991 which is incorporated herein by reference, beam location errors are known to be caused by errors in the angle of one or more of the facets 18, known as "pyramidal error", by wobble of the rotation axis 26 or by a combination of both sources of error.
Pyramidal errors are illustrated in FIGS. 2A and 2B, to which reference is now briefly made. FIGS. 2A and 2B are side views of the polygon 14.
Ideally, the reflecting surface of each facet 18 should be parallel to the rotation axis 26 such that the incoming beam 12 is reflected along a plane 30 perpendicular to the rotation axis 26.
However, typically, polygons 14 are manufactured with facets, labeled 32 and 34 in FIGS. 2A and 2B, respectively, at angles .alpha..sub.1 and .alpha..sub.2, respectively, to the ideal direction. An incoming beam which impinges an angled facet 32 or 34 is reflected along a reflection plane which is at an angle to plane 30 twice as large as the error in the facet direction. Thus, in FIG. 2A, the reflection plane 36 is at an angle 2.alpha..sub.1 to the plane 30 and in FIG. 2A, the reflection plane 38 is at an angle 2.alpha..sub.2 to the plane 30.
The wobble is a rotation of the rotation axis 26 of the spinner 14 and is caused by inaccurate balancing of the polygon. The wobble causes a changing angular error .alpha..sub.3, which, in turn, causes a changing error in the reflection plane in a manner similar to that described hereinabove for FIGS. 2A and 2B.
The scanning error is typically a combination of the two types of error and there are a plurality of methods by which prior art scanning devices reduce the scanning error.
Some prior art scanning devices include a cylindrical lens in the pre-spinner optical system 16. The cylindrical lens concentrates the laser beam into a line parallel to the scanning direction and focuses the concentrated beam onto the currently active facet 18.
However, in order to focus the reflected beam onto the medium 22, the cylindrical lens requires that the post-spinner optical system 20 be comprised of complicated optical elements, producing an expensive scanning device.
In other prior art devices, the spinner 14 is comprised of an element having two flat, opposing reflective surfaces, such as a penta-prism, from which the beam reflects. Due to the opposition of the surfaces, any errors in the laser beam caused by one surface are canceled by the reflection off the second surface.
This solution is typically utilized to reduce wobble in single faceted devices and is not easily extendible to a multi-faceted polygonal spinner.
Alternatively, in order to reduce the wobble, it is known to utilize expensive oil or air bearings for the spinner 14. If there is any pyramidal error, it is measured and a LookUp Table (LUT) is utilized to cancel it out.
Finally, it is known to include a beam position detector in the post-spinner optical system 20 and to include a reference beam, in addition to the scanning beam or beams, which follows a large percentage of the optical path of the scanning beams. The beam position detector detects the location of the reference beam, in order to continually measure the error in the location of the scanning beams. The measured data is provided either to acousto-optic deflectors for deflecting the reference and scanning beams to the correct location, or, as described in U.S. Pat. No. 5,247,174 assigned to the common owners of the present application, to piezo-electric crystals for shifting the location of fiber optic bundles which carry the scanning and reference beams to the correct location. The closed-loop control system thus produced maintains the scanning and reference beams in the desired locations.
However, the beam position detector is expensive, it requires that an additional beam, the reference beam, be produced and it includes complicated optics to ensure that the two beams follow almost identical optical paths.